1. Field of the Invention
This invention relates to optical communication systems and, more particularly, to an optical system which accommodates network traffic with high throughput and low latency and effects high-speed header detection and generation.
2. Description of the Background Art
Recent research advances in optical Wavelength Division Multiplexing (WDM) technology have fostered the development of networks that are orders of magnitude higher in transmission bandwidth than existing commercial networks. While such an increase in throughput is impressive on its own, a corresponding decrease in network latency must also be achieved in order to realize the Next Generation Internet (NGI) vision of providing the next generation of ultra high speed networks that can meet the requirements for supporting new applications, including national initiatives. Towards this end, current research efforts have focused on developing an ultra-low latency Internet Protocol (IP) over WDM optical packet switching technology that promises to deliver the two-fold goal of both high throughput with low latency. Such efforts, while promising, have yet to fully realize this two-fold goal.
There are a number of challenging requirements in realizing such IP/WDM networks. First, the NGI network must inter-operate with the existing Internet and avoid protocol conflicts. Second, the NGI network must provide not only ultra low-latency, but must take advantage of both packet-switched (that is, bursty) IP traffic and circuit-switched WDM networks. Third, it is advantageous if the NGI network does not depend upon precise synchronization between signaling and data payload. Finally, a desired objective is that the NGI network accommodates data traffic of various protocols and formats so that it is possible to transmit and receive IP as well as non-IP signals without the need for complicated synchronization or format conversion.
Comparison with Other Work
The Multi-Wavelength-Optical Network (MONET) system, as reported in the article xe2x80x9cMONET: Multi-Wavelength Optical Networkingxe2x80x9d by R. E. Wagner, et al. and published in the Journal of Lightwave Technology, Vol. 14, No. 6, June 1996, demonstrated a number of key milestones in optical network including transparent transmission of multi-wavelength through more than 12 reconfigurable network elements spread over the national scale fiber distance. The network, however, is circuit-switched and suffers inefficiency in accommodating bursty traffic. The typical connection setup time from request to switching is a few seconds, limited by capabilities of both Network Control and Management (NCandM) and hardware. Recent efforts within the MONET program to improve on the efficiency concentrated on the xe2x80x9cJust-in-Time signalingxe2x80x9d scheme. This method utilizes embedded 1510 nm NCandM signaling which precedes the data payload by an estimated delay time. This estimation must be accurately made for each network configuration for every wavelength in order to synchronize the signaling header and switching of the payload.
In accordance with the present invention, the optical packet header is carried over the same wavelength as the packet payload data. This approach mitigates the issue of header and payload synchronization. Furthermore, with a suitable use of optical delay at each intermediate optical switch, it eliminates the need to estimate the initial burst delay by incorporating the optical delay directly at the local switches. This makes a striking difference with Just-In-Time signaling in which the delay at each switch along the path needs to be known ahead of time and must be entered in the calculation for the total delay. Lastly, there is little time wasted in requesting a connection time and actually achieving a connection. In comparison to a few second delays seen in MONET, the present inventive subject matter reduces the delay to minimal, only limited by the actual hardware switching delays at each switch. The current switching technology realizes delays of only several microseconds, and shorter delays will be possible in the future. Such a short delay can be incorporated by using an optical fiber delay line at each network element utilizing switches. The present inventive subject matter achieves the lowest possible latency down to the fundamental limit of the hardware, and no lower latency can be achieved by any other technique.
The Optical Networks Technology Consortium (ONTC) results were reported in the article xe2x80x9cMultiwavelength Reconfigurable WDM/ATM/SONET Network Testbedxe2x80x9d by Chang et al. and published in the Journal of Lightwave Technology, Vol. 14, No. 6, June 1996. Both Phase I (155 Mb/s, 4-wavelength) and Phase II (2.5 Gb/s, 8-wavelength) of the ONTC program were configured on a Multihop ATM-based network. While such an ATM based architecture added a large overhead and excluded the possibility of a single-hop network, the packet/header signaling was made possible by utilizing the isochronous ATM cell itself. This communication of NCandM information is made through the same optical wavelength, potentially offering similar benefits as with the technique of the present invention. However, the inventive technique offers a number of significant advantages over the ATM-based signaling. First, the inventive technique offers a single hop connection for the payload without the need to convert to electrical signals and buffer the packets. Second, it offers far more efficient utilization of the bandwidth by eliminating excessive overheads. Third, it allows strictly transparent and ultra-low latency connections.
The DARPA sponsored All-Optical-Network (AON) Consortium results were reported in an article entitled xe2x80x9cA Wideband All-Optical WDM Networkxe2x80x9d, by I. P. Kaminow et al. and published in the IEEE Journal on Selected Areas of Communication, Vol. 14, No. 5, June, 1996. There were actually two parts of the AON program: WDM as reported in the aforementioned article, and TDM reported in a companion paper in the same issue. First the WDM part of the AON program is first discussed, followed by the TDM part.
The AON architecture is a three-level hierarchy of subnetworks, and resembles that of LANs, MANs, and WANs seen in computer networks. The AON provides three basic services between Optical Terminals (OTs): A, B, and C services. A is a transparent circuit-switched service, B is a transparent time-scheduled TDM/WDM service, and C is a non-transparent datagram service used for signaling. The B service uses a structure where a 250 microsecond frame is used with 128 slots per frame. Within a slot or group of slots, a user is free to choose the modulation rate and format. The B-service implemented on the AON architecture is closest to the IP over WDM which is the subject matter of the present invention. However, the separation of NCandM signaling in the C-service with the payload in the B-service requires careful synchronization between the signaling header and the payload. This requirement becomes far more stringent as the 250 microsecond frame is used with 128 slots per frame with arbitrary bit rates. Not only the synchronization has to occur at the bit level, but this synchronization has to be achieve across the entire network. The scalability and interoperability are extremely difficult since these do not go in steps with the network synchronization requirement. The present inventive subject matter requires only that the payload and the header are transmitted and received simultaneously, inter-operates with existing IP and non-IP traffic, and offers scalability.
TDM efforts are aimed at 100 Gb/s bit rates. In principle, such ultrafast TDM networks have the potential to provide truly flexible bandwidth on demand at burst rates of 100 Gb/s. However, there are significant technological challenges behind such high bit rate systems mainly related to nonlinearities, dispersion, and polarization degradations in the fiber. While the soliton technologies can alleviate some of the difficulties, it still requires extremely accurate synchronization of the networkxe2x80x94down to a few picoseconds. In addition, the header and the payload must have the identical bit rates, and as a consequence, bit-rate transparent services are difficult to provide. The subject matter in accordance with the present invention does not depend on precise synchronization, relies on no 100 Gb/s technologies, and offers transparent services.
The Cisco Corporation recently announced a product based on Tag-Switching and the general description of Cisco""s Tag-Switching is available at the worldwide-web site, (http://www.cisco.com/warp/public/732/tag/). Cisco""s (electronic) Tag Switching assigns a label or xe2x80x9ctagxe2x80x9d to packets traversing a network of routers and switches. In a conventional router network, each packet must be processed by each router to determine the next hop of the packet toward its final destination. In an (electronic) Tag Switching network, tags are assigned to destination networks or hosts. Packets then are switched through the network with each node simply swaps tags rather than processing each packet. An (electronic) Tag Switching network will consist of a core of (electronic) tag switches (either conventional routers or switches), which connect to (electronic) tag edge routers on the network""s periphery. (Electronic) Tag edge routers and tag switches use standard routing protocols to identify routes through the network. These systems then use the tables generated by the routing protocols to assign and distribute tag information via a Tag Distribution Protocol. Tag switches and tag edge routers receive the Tag Distribution Protocol information and build a forwarding database. The database maps particular destinations to the tags associated with those destinations and the ports through which they are reachable.
When a tag edge router receives a packet for forwarding across the tag network, it analyzes the network-layer header and performs applicable network layer services. It then selects a route for the packet from its routing tables, applies a tag and forwards the packet to the next-hop tag switch.
The tag switch receives the tagged packet and switches the packet based solely on the tag, without re-analyzing the network-layer header. The packet reaches the tag edge router at the egress point of the network, where the tag is stripped off and the packet delivered. After Cisco made its announcement about (Electronic) Tag Switching, the IETF (Internet Engineering Task Force) has recommended a MPLS (Multi-protocol Label Switching) to implement standardized, vendor-neutral (electronic) tag-switching function in routers and switches, including ATM switches.
A number of features in the Cisco""s (electronic) Tag Switching is similar to the Optical Tag Switching which is the subject matter of the present invention, with the features aimed at the similar goals of simplifying the processing required for packet routing. The key differences are as follows. First, the optical tag switching is purely optical in the sense that both tag and data payload are in an optical form. While each plug-and-play module (a component of the present inventive system) senses the optical tag, the actual packet does not undergo optical-to-electrical conversion until it comes out of the network. The Cisco""s (electronic) Tag Switching will be all electrical, and applies electronic detection, processing, and retransmission to each packet at each router. Secondly, the Optical Tag Switching of the present invention achieves lowest possible latency and does not rely on utilizing buffers. Electronic tag switching will have far greater latency due to electronic processing and electronic buffering. Thirdly, the Optical Tag Switching of the present invention utilizes path deflection and/or wavelength conversion to resolve blocking due to contention of the packets, whereas the Electronic Tag Switching will only utilize electronic buffering as a means to achieve contention resolution at the cost of increased latency, and the performance is strongly dependent on packet size. The present invention covers packets of any length. Lastly, the Optical Tag Switching of the present invention achieves a strictly transparent network in which data of any format and protocol can be routed so long as it has a proper optical tag. Hence the data can be digital of any bit rate and modulation formats. The Electronic Tag Switching requires that data payload to have the given digital bit rate identical to the electronic tag since the routers must buffer them electronically.
Another representative technology that serves as background to the present invention is the so-called Session Deflection Virtual Circuit Protocol (SDVC), which is based on deflection routing method. The paper entitled xe2x80x9cThe Manhattan Street Networkxe2x80x9d, by N. F. Maxemchukxe2x80x9d as published in the Proceedings on IEEE Globecom ""85, pp 255-261, December 1985, discusses that when two packets attempt to go to the same destination, one packet can be randomly chosen for the preferred output link and the other packet is xe2x80x9cdeflectedxe2x80x9d to the non-preferred link. This means that packets will occasionally take paths that are not shortest paths. The deflection method utilized by the present invention does not xe2x80x98randomlyxe2x80x99 select the packet to go to the most preferred path; rather, it attempts to look into the priorities of the packets, and send the higher priority packet to be routed to the preferred path. The packets will be deflected if they have lower priorities; however, both xe2x80x98path deflectionxe2x80x99 and xe2x80x98wavelength deflectionxe2x80x99 are utilized. The path deflection is similar to conventional SDVC in that the optical packet will be simply routed to the path of the next preference at the same wavelength. The wavelength deflection allows the optical packet to be routed to the most preferred path but at a different wavelength. This wavelength deflection is achieved by wavelength conversion at the network elements. Partially limited wavelength conversion is utilized, meaning not all wavelengths will be available as destination wavelengths for a given originating wavelength. The wavelength deflection allows resolution of blocking due to wavelength contentions without increasing the path delay. The combination of path and wavelength deflections offers sufficiently large additional connectivities for resolving packet contentions; however, the degree of partial wavelength conversion can be increased when the blocking rate starts to rise. Such scalability and flexibility of the network is not addressed by conventional SDVC.
Besides the foregoing overall system considerations elucidated above, there is also the issue of how to effectively detect and/or re-insert a header which is combined with a data payload for propagation over the network using the same optical wavelength. The primary focus in the literature has been on a technique for combining sub-carrier headers with a baseband data payload. The very first two articles addressing this issued were published in 1992 by A. Bidman et. al., who combined a 2.56 Gb/s data payload with a 40 Mb/s header on 3 GHz carrier [A. Budman, E. Eichen, J. Schalafer, R. Olshansky, and F. McAleavey, xe2x80x9cMultigigabit optical packet switch for self-routing networks with subcarrier addressing,xe2x80x9d Techical Digest, paper TuO4, pp.90-91, OFC""92], and W. I. Way et al., who combined a 2.488 Gb/s data payload with a tunable microwave pilot tone (tuned between 2.520 and 2.690 GHz) to route SONET packet in a WDM ring network via acousto-optical tunable filter [W. I. Way, D. A. Smith, J. J. Johnson, H. Izadpanah, and H. Johnson, xe2x80x9cSelf-routing WDM high-capacity SONET ring network,xe2x80x9d Technical Digest, paper TuO2, pp.86-87, OFC""92, and W. I. Way, D. A. Smith, J. J. Johnson, and H. Izadpanah, xe2x80x9cA self-routing WDM high-capacity SONET ring network,xe2x80x9d IEEE Photonics Technology Letters, vol.4, pp.402-404, April 1992.2,3]. Both of articles used a single laser diode to carry the data payload and sub-carrier header. This technique has also been extensively studied in a local-area DWDM optical packet-switched network [R. T. Hofmeister, L. G. Katzovsky, C. L. Lu, P. Poggiolini, and F. Yang, xe2x80x9cCORD: optical packet-switched network testbed,xe2x80x9d Fiber and Integrated Optics, vol.16, pp.199-219, 1997], and several other all-optical networks [E. Park and A. E. Willner, xe2x80x9cNetwork demonstration of self-routing wavelength packets using an all-optical wavelength shifter and QPSK subcarrier routing control,xe2x80x9d IEEE Photonics Technology Letters, vol.8, pp.938-940, 1996; and M. Shell, M. Vaughn, A. Wang, D. J. Blumenthal, P. J. Rigole, and S. Nilsson, xe2x80x9cExperimental demonstration of an all-optical routing node for multihop wavelength routed networks,xe2x80x9d IEEE Photonic Technology Letters, vol.8, pp.1391-1393, 1996].
Instead of combing a sub-carrier headers with the data payload in the electrical domain, they have also been combined in the optical domain by using two laser diodes at different wavelengths [B. H. Wang, K. Y. Yen, and W. I. Way, xe2x80x9cDemonstration of gigabit WDMA networks using parallel-processed sub-carrier hopping pilot-tone (P3) signaling technique,xe2x80x9d IEEE Photonics Technology Letters, vol.8, pp.933-934, July 1996].
However, using two wavelengths to transport data payload and header separately may not be practical in the following sense: in an all-optical DWDM network, it is preferred that the header, which may contain network operations information, travels along the same routes as data payload so that it can truthfully report the updated status of the data payload. If the header and the data payload were carried by different wavelengths, they could be routed in the network with entirely different paths, and the header may not report what the data payload has really experienced. Therefore, although it is preferred that the sub-carrier header and the data payload be carried by the same wavelength, the art is devoid of such teachings and suggestions.
The sub-carrier pilot-tone concept presented in Wang et al. was extended to multiple pilot tones by Shieh et al. [W. Shieh and A. E. Willner, xe2x80x9cA wavelength routing node using multifunctional semiconductor optical amplifiers and multiple-pilot-tone-coded subcarrier control headers,xe2x80x9d IEEE Photonics Technology Letters, vol.9, pp.1268-1270, September 1997.], mainly for the purpose of increasing the number of network addresses.
Recently, consideration has been given to xe2x80x98header replacementxe2x80x99 for the high-throughput operation in a packet-switched network in which data paths change due to link outages, output-port contention, and variable traffic patterns. Moreover, header replacement could be useful for maintaining protocol compatibility at gateways between different networks. However, the only method which has been reported is for time-division-multiplexed header and data payload requires an extremely high accuracy of timing synchronization among network nodes [X. Jiang, X. P. Chen, and A. E. Willner, xe2x80x9cAll optical wavelength independent packet header replacement using a long CW region generated directly from the packet flag,xe2x80x9d IEEE Photonics Technology Letters, vol.9, pp.1638-1640, November 1998].
From this overview of the art pertaining to details of header generation and detection, it is readily understood that the art is devoid of teachings and suggestions wherein sub-carrier multiplexed packet data payload and multiple sub-carrier headers (including old and new ones) are deployed so that a  greater than 2.5 Gbps IP packet can be routed through a national all-optical DWDM network by the (successive) guidance of these sub-carrier headers, with the total number of sub-carrier headers that can be written is in the range of forty or more.
The present invention utilizes a unique optical signaling header technique applicable to optical networks. Packet routing information is embedded in the same channel or wavelength as the data payload so that both the header and data information propagate through the network with the same path and the associated delays. However, the header routing information has sufficiently different characteristics from the data payload so that the signaling header can be detected without being affected by the data payload and that the signaling header can also be stripped off without affecting the data payload. The inventive subject matter allows such a unique signal routing method to be overlaid onto the conventional network elements in a modular manner, including the insertion, detection and processing of the optical header.
In accordance with a broad method aspect of the present invention commensurate with the overall NGI system, a method for propagating a data payload from an input network element to an output network element in a wavelength division multiplexing system composed of a plurality of network elements, given that the data payload has a given format and protocol, includes the following steps: (a) adding a header to the data payload prior to inputting the data payload to the input network element, the header having a format and protocol and being indicative of the local route through each of the network elements for the data payload and the header, the format and protocol of the data payload being independent of the format and protocol of the header; and (b) detecting the header at each of the network elements as the data payload and header propagate through the WDM network, wherein the header is composed of one or more header signals each being conveyed by a distinct sub-carrier frequency and arranged so that the highest detectable sub-carrier frequency corresponds to an active header signal, the plurality of sub-carrier frequencies occupying a frequency band above the data payload, such that the step of detecting includes the steps of (i) concurrently measuring the header signals to produce a header selection signal, (ii) selecting the active header signal as conveyed by the highest detectable sub-carrier frequency under control of the header selection signal, and (iii) processing the header to delete the header signals and recover only the data payload; moreover, the method further includes the following steps after the step of detecting: (iv) determining a new active header signal, and (v) inserting the new active header signal in place of the deleted header signals.
In accordance a broad system aspect of the present invention for an individual network element, a system for replacing a header propagating with an data payload at a given optical wavelength, the header being composed of one or more header signals each being conveyed by a distinct sub-carrier frequency and arranged so that the highest detectable sub-carrier frequency corresponds to an active header signal, the plurality of sub-carrier frequencies occupying a frequency band above the data payload, includes the following components: (a) a measurement device for concurrently measuring the header signals to produce a header selection signal; (b) a selector, coupled to the measurement device, for determining the active header signal as conveyed by the highest detectable sub-carrier frequency under control of the header selection signal to route the header and payload; (c) a processor, responsive to the header, for deleting the header signals and recovering only the data payload; (d) a header generator, responsive to the selector, for determining a new active header signal; and (e) means, responsive to the header generator, for inserting the new active header signal into the header in place of the deleted header signals.